fix #25, terminate the pasted string with NUL

Allocate one more byte, and append a '\0'.

Whoops.

interface.cpp

@@ -1126,8 +1126,9 @@ struct apple2screen : public widget
             fseek(fp, 0, SEEK_END);
             long length = ftell(fp);
             fseek(fp, 0, SEEK_SET);
-            char *text = (char *)malloc(length);
+            char *text = (char *)malloc(length + 1);
             fread(text, 1, length, fp);
+            text[length] = '\0';
             event_queue.push_back({PASTE, 0, text});
             fclose(fp);
             return true;